This invention relates generally to regulated power supplies, and particularly relates to a power supply for use in a video display terminal.
A typical video display terminal includes a power supply, a monitor, a keyboard and logic circuitry. The user provides inputs via the keyboard and receives visual feedback from the monitor. The power supply generally energizes all video terminal components and the digital logic circuitry permits the terminal to operate with various other systems including a digital computer.
Prior art video display terminal power supplies are generally either of the linear regulator type or of the switch mode type. A linear regulator typically generates many B+ voltages from a power transformer and provides linear regulation for each of these B+ voltages. This approach is expensive because of the potentially large number of regulators required and is inefficient from a power utilization standpoint. In addition, the large power transformer required generates a substantial amount of heat which must be eliminated and the inductive leakage of this large transformer can easily degrade video imagery. The other approach to energizing a video display terminal involves a switching mode power supply in which a single voltage regulator is used with various conductive modes selectively switched in to provide various DC output signals. The problem with this approach is that the switching of all of the non-linear regulators occurs during the video display time interval producing display distortions during switching periods. From a more practical standpoint, in the past the video display terminal designer was required to integrate the power supply with the other terminal components which involved both analog and digital design considerations. With some terminal components being primarily digital in nature and others primarily analog, this lack of a systems integration approach in video display terminal design added to their complexity, cost and limited flexibility in meeting a wide variety of applications.
U.S. Pat. No. 3,818,128 to Chambers et al discloses a switching mode power supply which claims to eliminate undesirable interference of the power supply with the display system by operating the power supply at a substantially higher frequency than the line sweep frequency and which is synchronized therewith so that the power supply operates at a harmonic of the sweep frequency. By thus synching the video display power supply frequency with the display logic so that power switching occurs only during intercharacter time intervals any noise spike produced occurs during an intercharacter time interval when the cathode ray tube is blanked thus eliminating interference with the display since switching never occurs during display time. While this invention reduces switching mode video display interference, it is designed to operate at extremely high frequencies, i.e., between 1 and 100 kilohertz, and is too complex and costly for the typical video display terminal installation. This system is also limited to the display of white letters on a black background to avoid switching during video display "on" time.
Another approach to video display power supply design is disclosed in U.S. Pat. No. 3,641,267 to Cavallari wherein is described a power supply designed for stepping down an AC or DC input source to a stable DC output level. In this power supply a chopper circuit incorporating one or more normally blocked transistors is inserted between the input circuit and a load circuit. Voltage stabilization of the output signal is achieved by incorporating a voltage-limiting means such as a Zener diode in the input circuit which limits the amplitude of the pulsed output of the input transistor and hence the voltage level of the power supply output signal. With a transformer incorporated in the network and the Zener diode coupled to a secondary winding of the transformer the current flowing through the primary winding will be controlled by the Zener in that excess secondary current is dissipated through the Zener diode connected between the base and the emitter of the associated transistor. The Cavallari invention is intended primarily to permit a large variety of DC level output signals to be produced from large variations in the AC or DC input voltage level and is not designed to improve video display quality by synchronization techniques or precise output voltage level regulation. Still another video display power supply is disclosed in U.S. Pat. No. 3,629,497 to Soardi et al which is limited to generating a low voltage DC output signal.
The present invention is intended to avoid the aforementioned computer terminal and video display power supply problems by providing an isolated, well-regulated, source of low impedance DC output signals which are synchronized with video display sweep circuitry and which can be used to drive various video display logic levels and computer terminal subsystems.